Method of producing coated casting surfaces and casting surfaces for producing coated paper



Aprll 23, 1963 J. R. WIEBUSH 3,086,883

METHOD OF PRODUCING COATED CASTING SURFACES AND CASTING SURFACES FOR PRODUCING COATED PAPER Filed Jan. 24, 1957 Solid Snhcone Qesm Cashnq SurFace ApproxOfi-L'fi Alkyl Groups Per Silicon Atom Fig.1

2 Metal $ubstrat Fig Coat Uncomam'mated Metal Substrafe Surface Wurh SOlUtIOr] OF Silicone Ream.

Dry Qesm /B Heat Treat Qes'ln TO Form fc Cross-Lmked P0\9mer5 INVENTOR.

JOSEPH E. W/EEUSH BY w $2 States This invention relates to the manufacture of a highly glossed finish cast-surfaced, mineral-coated paper and more particularly to casting surfaces therefor.

A primary object of this invention is to provide an improved casting surface for producing cast-surfaced coated paper, said surface affording excellent release of 'mineral coatings during continuous cast coating operations at the same speeds of operation, or in excess of those obtainable by known methods of the prior art.

Another object of this invention relates to the development of a paper product distinguished by its extremely high gloss which substantially exceeds that of presently known cast-surfaced mineral-coated paper products.

Other and more specific objects, as well as the novel .characteristics of the casting surfaces of this invention which contribute to functional improvement, will appear hereinafter.

1 Heretofore, mineral coated paper has been provided .with a glossy finish by various methods. One of these methods involves placing the surface of the'paper, wet

with the coating material, against the surface of a heated chromium-plated drum or endless belt to which the wet paper adheres. As the drum revolves, the water in the coating material evaporates and, theoretically, when the coating material is sufficiently dry, the paper no longer adheres to the drum so that it can be led away to a winding roll. Thus, by this method, it was thought not necessary?) calender cast coated paper, because it remained in contact with the polished surface of the chromium-plated drum or endless belt until the coating material was dry, with the expected result that the coated paper was to have the luster of the chromium drum or I endless belt surface. However, the product made in this manner Was subject to the objection that when the coating material had dried, the coated paper was not completely released from the casting surface, particles of coating would stick to the chromium surface, thereby damaging the quality of the paper with surface defects and roughening the surface of the chromium drum or belt.

Another method has been proposed for producing highgloss cast coated paper, which relates essentially to the application of a mineral coated web to a chromium drum or endless belt casting surface which is treated with oleaginous film-forming materials such as relatively waterinsoluble liquids which wet the casting surface and spread into a continuous film, yet will not vaporize too readily at operating temperatures. The coating on the web is intended to adhere firmly to the casting surface until the coating has dried to a substantially non-plastic condition and. is thereafter released from the chromium casting surface. Satisfactoryresults utilizing this method allegedly are obtained only by periodic or continuous application of the oleaginous compound tothe casting surface. Obviously, the product of this method is subject to the objection that the required function of oleaginous mate- .rial on thechromium casting surface is necessarily dependent on and the exact quantity thereof determined by a skilled operator who watches the point at which the paper leaves the chromium casting surface, and hopefully increases or decreases the percentage of liquid oleaginous material on the chromium drum and/or in the coating ice color in order toobtain adequate release of the coated paper from the chromium casting surface. As a result, the paper industry has long been aspiring to make avail able a high-gloss paper product, the dried coating of which can be satisfactorily and effectively released from a casting surface without dependence on the skill of an operator, and which is sufficiently cheap, adapting it to large-scale usage.

Certain properties have been recognized as being required of casting surfaces, if they are to be successfully employed as release casting surfaces in the production of high-glossed finish cast surfaced, mineral coated paper. For example, they should be wetted by but effect a condition of non-adhesion between the fluid and subsequently dried mineral coating on the paper web and the casting surface on to which the fluid coating is applied and dried thereon. And it is now known that this condition requires a substantially non-polar casting surface. Metal casting surfaces are in general polar and in order to prevent the development of an adhesive bond with a mineral coating material as applied to a paper Web, these surfaces should in some way be changed to cause reduced or substantial non-polarity so as not to develop an adhesive bond with the mineral coating material during casting operations. As noted above, an attempt-has been made to accomplish this requirement by the use of oleaginous liquids. And it has been found that certain oleaginous liquids under critical conditions could possibly accomplish this aforementioned reduction in polarity. However, the oleaginous liquids which are most effective have long chainmolecules with a polar grouping which orients on a metal surface to leave a non-polar grouping which provides a separating plane. Moreover, such oleaginous liquids as films are easily destroyed and must be continuously applied to a casting surface, as taught by prior methods, in order not to interrupt continuous casting operations. In addition, oleaginous liquids possessing no polar group, or only a Weak polar group, or wherein the polar group is prevented from association with the casting surface because of stereo-chemical limitations, are substantially completely ineffective in producing the required condition of non-adhesion between the dried coating and the casting surface.

In the drawing which shows an embodiment of the present invention, FIGURE 1 is a diagrammatic illustration showing a metal substrate in section, which has been provided with a silicone casting surface according to the invention, and FIGURE 2 is a flow diagram illustrating the process of making a silicone casting surface in accordance with the present invention.

In accordance with the present invention, the desired non-adhesive requirement for cast coating has been unexpectedly provided by solid layers of high cohesive strength, which layers have been found to be durable and effectively useful over long periods of operation. Considering the invention generally, solid materials which can be for-med into self-supporting films which are nonpolar, or even weakly polar on their surface, will provide a suit-able casting surface with the required nonadhesive properties mentioned above, when supported on a metal or other substrate which has a substantially smooth surface. Examples of such materials, set forth hereinafter, when deposited from solution, or by other suitable means, orient on the metal or other substrate, with a polar portion of the molecule being adhesively attracted to the substrate and a non-polar portion of the material oriented at the exposed surface thereof. Since these materials have a high molecular weight, and have many chemical cross-linked bonds between adjacent molecules, they exist as solid films which are r'elatively permanent and are useful over extended periods of time.

:In contradistinction, when attempts are made to maintain a film of liquid oleaginous material on a casting surface such as a chromium drum, such a film can be provided only because of the influence of the metal substrate and, if the substrate were to be removed, the oleaginous film would collapse into a liquid because there is no cross-linl ing by chemical bonds.

The difference in the orientation of so-called oleaginous film forming materials and the silicone resin films of the present invention is readily perceptible from the following observation, which is predicated on the recognized principle that orientation of individual molecules is produced primarily by the unsymmetrical electrical charge of the molecule due to the polar-nonpolar portions of the molecule. With respect to oleaginous materials defined in the art as any relatively water-insoluble liquid which will wet a previously prepared casting surface and spread into a continuous film, yet will not vaporize too readily at operating temperatures between 130 and 210 F., such materials, while tending to promote orientation may be held together by weak Van der Waals forces between the nonpolar parts of the molecules. And if orientation is effected, the long chain oleaginous molecules orient with the length of the chain at an appreciable angle to a chromium substrate. The length of the oleaginous molecule has very little effect on the area of surface covered by the oleaginous film and such film collapses into a liquid if the substrate is removed. In connection with the silicone resin films of the present invention, the polar part of the silicone resin film is the oxygen atom and orientation of the silicone resin film is brought about by the inherent tendency of the oxygen atoms to line up adjacent to a metal or other substrate. Since the polar part of these molecules, or more precisely, the polymer film, is in the center of each monomer unit, these molecules have the propensity of lying parallel to the substrate and- -are bound to the metal or other substrate at these polar centers. After orientation, the silicone resin films of this invention are held together by chemical bonds. Thus, the film strength of these silicone resin films, which exist as a solid film, i-s outstandingly stronger than the strength of oleaginous films and such silicone resin films will not collapse if the substrate were to be removed.

Now, considering the invention in greater detail, reference will be first made to certain organo-siloxane resins which have been found to provide a casting surface for the production of highly glossed finish cast-surfaced, mineral coated paper whereby a condition of non-adhesion is established between the mineral coating on the paper and the casting surface. More specifically, the casting surface of this invention may be identified chemically as a film of cross-linked polymers of certain alkyl siloxanes, wherein the ratio of alkyl groups to silicon atoms is less than 2, and preferably in the range of 0.8 to 1.3. And the structural formulae provisionally adopted for this class of compounds are:

or R SiO, wherein R is alkyl, preferably methyl or ethyl and x is 0.8-1.3. Moreover, in the thermoset crosslinked polymer molecules as they exist in the casting surface of this invention, the number of monomer units is very high and is believed to be in excess of 2000 per molecule.

These compounds may be broadly identified chemically as methyl silicone resins and are available commercially under various identification numbers such as Dow Corning Fl2l, G-E Silicone Resin SR 53, or the like, which, as well as can be determined, are essentially variations of methyl silicone and its polymers so that when the silicone resin in a solvent and in solution is applied to a metal or other surface and is subjected to a polymerizing treatment such as a heat treatment, a chemical reaction results whereby the polymers are joined by chemical bonds and become three-dimensional. As a result of the polymerizing treatment, a solid film is formed on the metal or other surface and adheres thereto. This film has cohesive strength due to the chemical bonding and when it is possible to remove it from the metal, an integral film is obtained.

The term methyl silicone resin as used generally herein and in the appended claims is intended to include within its meaning chemical compounds, specifically condensation products and polymers of varying high molecular weight, composed essentially of silicon, oxygen and at least one alkyl group, which is preferably a methyl or ethyl group, attached directly to the silicon. The methyl or ethyl group or groups are attached to the silicon atoms contained in the molecule. Ordinarily, the polymer will have in its molecule an average of approximately one to approximately two alkyl groups for each silicon atom. And, as pointed out hereinbefore, a preferred material forming the casting surface of this invention is a polymer containing an average of approximately 0.8 to approximately 1.3 alkyl groups per silicon atom.

The alkyl silicones of this invention are not liquids, but are resinous thermosetting polymers. And it has been found that in their final form as an integral film on a metal or other surface, they have the outstanding characteristic of thermal stability and in addition are highly resistant to decomposition. In consequence, it is now possible to provide a casting surface for producing cast-surfaced miner-a1 coated paper, which casting surface has the outstanding characteristic of releasing mineral coatings without the appearance on the highly glossed finished paper of megoscopic or microscopic surface defects such as trenches, crevices, holes, stipple or the like.

Prior methods for manufacturing cast-surfaced, mineral-coated paper are directed to minimizing a variety of the aforementioned surface defects. For example, by the procedure given in US. Patent No. 2,214,565 it is alleged that such surface defects in cast-coated paper may be reduced if the mineral coating on the paper is made up of two or more layers instead of a single layer of mineral coating. Another exemplary procedure given in Us. Patent No. 2,214,564 for minimizing such surface defects suggests increasing the ratio of adhesive to pigment in the mineral coatings. However, it has now been found that such surface defects in cast-surfaced, mineralcoated paper are unexpectedly, inherently and substantially overcome when the organo-silox-ane resins of the present invention provide the casting surface whereby non-adhesion between the mineral coating on the paper and the casting surface is established. Conversely, many surface defects of a metal, or other substrate, are substantially masked by the organo-siloxane resin casting surface of the present invention.

In order to point out more fully the nature of the present invention, any suitable metal or other substrate is first cleaned so that it is substantially free of any contamination on its surface. For example, if the outer met-a1 surface of a revolving Monel metal, stainless steel or chromium-plated drum is used as the substrate, it has been found that such surfaces may be adequately cleaned by scrubbing witha paste of amorphous aluminum-oxide powder and a 20% solution of sodium carbonate in the manner in which a scouring powder is used, left to dry and rubbed off. In any event, the cleaning method is dependent upon the nature of the surface contamination to be removed.

A preferred methyl silicone resin in solution is then applied in any suitable manner to the cleaned substrate surface, and in order to obtain the desired coating, the resin after application to the metal surface, is allowed to dry by evaporation of the solvent. After drying and buffing, the resin is subjected to a polymerizing heat treatment at approximately 100 C. for about 3 to 4 hours, resulting in a chemical reaction by which crosslinked polymers are formed and are joined to the metal substrate by chemical bonds, thereby forming a solid film in integral adherence to the substrate.

Among the solvents which have been found suitable for bringing the silicone resins in solution for application to a metal or other substrate are included amyl acetate, benzene, carbon tetrachloride, cyclohexane, ethyl ether, gasoline, methylene chloride, mineral seal oil, V.M. and P. naphtha and Stoddard solvent.

The ratio of silicone resin to solvent prior to the application of the resin to the cleaned substrate surface is preferably adjusted to at least silicone content, and a small amount of catalyst, if desired, may be added to the solution before such application, to accelerate the cure of the silicone resin. Highly suitable casting surfaces have been obtained when the silicone resin solution as applied to a metal substrate consisted of 8 parts Dow Corning F-1 21 silicone resin, 1 part catalyst solution of lead 2-ethyl hexoate containing a 5% concentration of lead, and 12 parts V.M. and P. naphtha; the catalyst solution being available commercially as Dow Corning XY-26. As in the before described general process of this invention, the silicone resin in naphtha solution was applied as a thin continuous fil-m to a chromium drum substrate and was allowed to air dry to evaporate the solvent. Such air drying was followed by an ordinary buffing operation to polish the silicone resin, after which this resin film was subjected to a polymerizing heat treatment by heating the chromium drum substrate at approximately 100 C. for about /2 hour during which time a chemical reaction resulted by which cross-linked polymers Were formed and were joined to the chromium drum substrate by chemical bonds to form a solid silicone film casting surface which was in integral adherence to the chromium substrate. This resulting continuous, hard and solid silicone film was again polished by buffing to provide the outstanding casting surface of this invention and was used during continuous cast-coating operations to produce a mineral coated paper product which was characterized by extremely high gloss and was substantially free of surface defects.

The thickness of this solid casting surface film is believed to be in excess of 200 millimicrons and in other modifications of the invention, two or more of such films were superimposed on a metal substrate to provide a casting surface of indefinite longevity in the production of cast-coated paper, which surface, as well as the single layer solid film casting surface, provided excellent release of mineral coatings during continuous cast coating operations at relatively high speeds of operation.

Moreover, it has been found that the casting surfaces of the present invention during continuous cast coating operations afford excellent release of fluid coating compositions known in the paper coating art and when used with such coating compositions, produce cast-surfaced coated paper substantially free of surface defects.

Referring to the drawing which diagrammatically shows an embodiment of the present invention, FIGURE 1 shows a metal drum used as the substrate and having a solid silicone resin casting surface 1 for making cast-surfaced miner-al coated paper. The casting surface is obtained as illustrated in the fiow diagram of FIGURE 2 by applying to the adequately cleaned outer metal surface of the substrate 2, the silicone resin G-E Silicone Resin SR 53, supra, in a suitable solvent and in solution as a thin continuous film (step A). After air drying (step B) to evaporate the solvent, the resin film is subjected to a polymerizing heat treatment until a chemical reaction results by which cross-linked polymers are formed and joined to the outer surface of the metal substrate by chemical bonds (step C), thereby producing a solid substantially non-polar silicone film in integral adherence to the substrate. Thereafter, this solid silicone film is polished by buffing (step D) to provide the casting surface of this invention.

While the products and processes herein described are for the purpose of illustration only, it is to be understood that the present invention includes all modifications and equivalents which fall within the scope of the invention which is defined by the appended claims.

What is claimed is:

l. A process of making a casting surface for the manufacture of cast-surfaced mineral coated paper which comprises applying a methyl silicone resin in solution to a metal substrate surface, said resin having the formula R SiO where R is alkyl and the ratio of alkyl groups to silicon atoms is in the range of 0.8 to 1.3, drying the solvent of said solution and after-Wards subjecting the resin to a polymerizing heat treatment whereby a chemical reaction results to produce three-dimensional polymers, said polymers being joined to said substrate forming a solid film in integral adherence to said substrate.

2 A process of making a casting surface for the manufacture of cast-surfaced mineral coated paper which comprises applying a methyl silicone resin in solution to a metal substrate surface, said resin having the formula R SiO \where R is alkyl and x is in the range of 0.8 to 1.3, drying the solvent of said solution and afterwards subjecting the resin to a polymerizing heat treatment at approximately C. whereby a chemical reaction results to produce three-dimensional polymers, said polymers being joined to said substrate forming a solid film in integral adherence to said substrate.

3. A process for the production of a casting surface for making highly glossed finish cast-surfaced mineral coated paper by bonding a heat stable silicone composition to a metal drum surface which comprises coating the met-a1 surface with a solution in volatile solvent of a methyl silicone resin, said resin having the formula R SiO where R is alkyl and the ratio of alkyl groups to silicon atoms is less than 2.0; evaporating off the solvent to form a solid continuous cohesive film and bufling said film toa high gloss.

4. A casting surface for the production of highly glossed finish cast-surfaced, mineral coated paper and subject to casting temperatures in the range of 80 to C. com prising a structure having thereon a coating consisting essentially of a methyl silicone resin chemically bonded to a metal substrate to produce a solid integral film of substantially permanent character, said resin having the formula R SiO where R is alkyl and x is less than 2.0.

5. The process of making a casting surface for the production of cast-surfaced mineral coated paper which comprises coating a metal drum substrate surface with a solution in volatile solvent of a methyl silicone resin, said resin having the formula R SiO where R is alkyl and x is 0.84.3, evaporating the solvent of said solution to form a continuous resin film, subjecting the resin film to a polymerizing heat treatment to produce three-dimensional polymers, said polymers being integrally joined to said drum surface and forming a substantially non-polar exposed solid film, thereafter bufling said solid film to provide a polished casting surface, wherefrom ready release of mineral coating compositions in the production of said paper is obtained.

6. The process of making a casting surface for the production of cast-surfaced mineral coated paper which comprises coating a metal drum substrate surface with a solution in volatile solvent of a methyl silicone resin, said resin having the formula R SiO where R is alkyl and x is 0.84.3, evaporating the solvent of said solution to form a continuous resin film, subjecting the resin film to a polymerizing heat treatment until a chemical reaction results by which cross-linked polymers are formed and joined to the outer surface of said substrate by chemical bonds thereby producing a solid substantially non-polar film in integral adherence to the substrate, and thereafter buffing said solid film to provide a polished casting surface, wherefrom ready release of mineral coating compositions in the production of said paper is obtained. 5

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES 1957 Guide to Dow Corning Silicone Products, page 4, 1956. 

1. A PROCESS OF MAKING A CASTING SURFACE FOR THE MANUFACTURE OF CAST-SURFACED MINERAL COATED PAPER WHICH COMPRISES APPLYING A METHYL SILICON RESIN IN SOLUTION TO A METAL SUBSTRATE SURFACE, SAID RESIN HAVING THE FORMULA 